The prior art discloses systems for detecting defects and/or irregularities on a web of material being conveyed past along a measurement plane and comprising an apparatus for emitting linear light. The term “linear light” is to be understood here as meaning as bundled light that produces an illuminated line on the web of material, usually extending transversely to the web. A system of this kind can be used to inspect for example a web of paper that is being conveyed past along the measurement plane at high speed, preferably employing one or more line scan cameras to locate defects and/or irregularities (e.g. thin spots, inclusion of flies or dirt particles, etc.) by detecting differences in brightness on the web of material by the direct lighting method or the backlighting method. In the direct lighting method, both the emission apparatus and the camera are disposed on the same side of the web of material, and in the backlighting method they are on different sides of the web.
Apparatuses for emitting linear light are known for example from WO 2004/006560 and U.S. Pat. No. 6,880,952 B2.